The ever durable A List Apart had a great article on Paper Prototyping today. I have to second the recommendation. I had a tremendously productive prototyping discussion with one of our engineers recently using nothing but sticky notes on a whiteboard. Software is great but sometimes the physicality of being able to move stuff around makes a big difference to your creativity.
The practice reminds me of Voice of the Customer, the marketing practice by which customer utterances are written on Post Its and grouped to identify customer pain areas and requirements. Very low tech but very effective.